I am a new comer here and I am amazed by the quality of what I read. Congratulations Ryan and others.
I really think that the GNSS/IMU integration through APX-18 and PostPac is an obstacle to large diffusion.
So my 2 cents :
1. I saw a promising software alternative here : https://github.com/rodralez/NaveGo
2. Ardusimple is also a good source for GNSS units, with Ublox F9P inside (up to 3 in one board, so pos, pitch roll yaw in once) https://www.ardusimple.com/simplertk2b-receivers/